1. Technical Field
The present disclosure relates to a LED driving circuit, in particular, to a LED driving circuit and method.
2. Description of Related Art
The light emitting diode (LED) has become widely used to various displays. Generally, the LED display to define n-bit grayscale means that the grayscale period T is divided into 2n or (2n−1) grayscale steps, and each grayscale step has a time interval T1, wherein T1=T/(2n) or T/(2n−1). A value of n-bit grayscale signal D [n−1:0] (referred to as the brightness value) is used to determine how many grayscale steps need to be turn-on (turn-on steps) in one grayscale period to determine the brightness. The grayscale period T may be equal to the frame period Tf, and the frame period Tf may also include the non-turn-on time Toff, wherein Tf=T+Toff.
The refresh rate has become more important because of the rapid development of displays. The turn-on time in one grayscale period can be divided to increase the refresh rate. For example, as shown in FIG. 2A, the conventional current output of 8 turn-on steps is continuous in one grayscale period when the brightness value is D [3:0]=1000. Please refer to FIG. 2B. When the current output of 8 turn-on steps are divided into 4 time intervals during the grayscale period, the refresh rate becomes 4/T and therefore has a fourfold increase compared with the conventional refresh rate. (the refresh rate of the conventional waveform is 1/T).
Conventionally, the driving circuit only provides one constant current I. When the brightness value is lower than the amount of time intervals, the refresh rate cannot be sustained. As shown in FIG. 1, when the brightness value is D [3:0]=0001, the turn-on time cannot be divided because there is only one turn-on step. Thus the refresh rate cannot be sustained.